With the advance of LSI technology, the memory capacity of IC chips reaches an order of mega bits and a rule of submicron is thus required for the fineness of wiring pattern. Accordingly, lithography light sources are shifting towards shorter wavelengths from the ultraviolet band to the far ultraviolet band which is more advantageous for fine patterning. Also, the current technique used for etching in the LSI manufacturing process is dry etching using RF plasma.
In these circumstances, the lithography technology uses resists that must be sensitive and transmissive to the wavelength used and resistant against dry etching. More particularly, novolak type resins and other aromatic resins are useful resists in photolithography, especially in G and I rays lithography, in view of their light transmittance at the wavelength used and plasma etching resistance.
However, the light source intensity of far ultraviolet light, including the mercury line spectrum which has a shorter wavelength than G and I line and excimer laser light such as KrF and ArF, is much weaker than the light source intensity of G and I line. Utilization of such ultraviolet light in photolithography has the problem wherein prior art resists conventionally used with G and I line provide unacceptable exposure sensitivity and low light transmittance to far ultraviolet light. As a consequence, there is a need for a new type of resist.
Resists of the chemically amplified type were developed as a substitute for the conventional resists. For example, Japanese Patent Application Kokai (JP-A) No. 45439/1984 discloses a resist composition comprising p-tert-butoxycarbonyloxy-.alpha.-methylstyrene, which is an acid unstable polymer having recurring branched groups and a diaryl iodonium salt, which is a photopolymerization initiator capable of generating acid upon exposure to radiation. Upon exposure of the resist to far ultraviolet light, the diaryl iodonium salt decomposes to generate an acid which in turn, causes cleavage of the p-tert-butoxycarbonyl group p-tert-butoxy-carbonyloxy-.alpha.-methylstyrene into a polar group. Then a desired pattern is obtained by dissolving away the thus exposed areas or unexposed areas with base or non-polar solvent.
JP-A 115440/1987 discloses a resist composition comprising poly-4-tert-butoxy-.alpha.-styrene and di(tert-butylphenyl) iodonium trifluoromethane sulforonate, which are dissolved in diglyme and then exposed to far ultraviolet light. This resist achieves a high degree of resolution through the same reaction mechanism as the resist of JP-A 45439/1984.
The polymers used in these well-known resists are obtained by conventional radical or cationic polymerization of corresponding monomers. Since no special attention is paid to the molecular weight distribution of polymers in the conventional polymerization techniques, polymers having a broad and uneven molecular weight distribution are obtained. According to the inventors' investigations, if a resist based on a polymer having a broad molecular weight distribution is used, a low molecular weight moiety of the polymer can be gasified during the vacuum step of the wafer manufacturing process, resulting in a lowering of vacuum and contamination of the processing atmosphere. There always arises the problem that the polymer locally experiences a fast or slow dissolution rate due to uneven molecular weight distribution. As a result, an unstable state of patterning will often occur.
In order to utilize the polymers obtained by conventional polymerization methods as resists, it is necessary to adjust the molecular weight distribution of the polymer after polymerization. Such post adjustment is cumbersome and effective control of the molecular weight is impossible to achieve. The conventional resist compositions are not regarded as suitable for the manufacture of high performance semiconductor elements such as super LSI. As a consequence, there is a desire to overcome these problems.